1. Field of the Invention
The present invention relates generally to a cutting insert for a rock drill bit useful in drilling subterranean boreholes and, in one or more embodiments, to such a cutting insert that significantly reduces the mechanical specific energy expended to extrude crushed rock particles across the face of a polycrystalline diamond cutting insert thereby effectively increasing the efficiency of a rock drill bit during drilling a subterranean borehole.
2. Description of Related Art
In the production of fluid, from subterranean environs, a borehole may be drilled in a generally vertical, deviated or horizontal orientation so as to penetrate one or more subterranean locations of interest. Typically, a borehole may be drilled by using drill string which may be made up of tubulars secured together by any suitable means, such as mating threads, and either a fixed cutter type or a roller cone type rock drill bit secured at or near one end of the drill string. Drilling operations may also include other equipment, for example hydraulic equipment, mud motors, rotary tables, whipstocks, as will be evident to the skilled artisan. Drilling fluid may be circulated via the drill string from the drilling rig to the rock drill bit. The drilling fluid may entrain and remove cuttings from subterranean rock face adjacent the rock drill bit and thereafter may be circulated back to the drilling rig via the annulus between the drill string and borehole. After drilling, the borehole may be completed to permit production of fluid, such as hydrocarbons, from the subterranean environs.
As drilling a borehole is typically expensive, for example up to $500,000 per day, and time consuming, for example taking up to six months or longer to complete, increasing the efficiency of drilling a borehole to reduce cost and time to complete a drilling operation is important. Historically, drilling a borehole has proved to be difficult since an operator of the drilling rig typically does not have immediate access to, or the ability to make decisions based upon detailed rock mechanical properties and must rely on knowledge and experience to change those drilling parameters that are adjustable. Where a drilling operator has no previous experience in a given geological area, the operator must resort to trial and error to determine the most favorable settings for those adjustable drilling parameters. Processes have been proposed which utilize a traditional calculation of mechanical specific energy (MSE), which is the summed total of two quantities of energy delivered to the subterranean rock being drilled: torsional energy and gravitational energy, and manual adjustment of drilling parameters as a result of such calculation in an attempt to increase drilling efficiency. The original calculation developed by Teale, R. (1965) is as follows:MSE=(Wb/ Ab)+((120*π*RPM*T)/(Ab* ROP))Where:                MSE=Mechanical Specific Energy (psi)        Wb=Weight on Bit (pounds)        Ab=Surface area of the bit face, or borehole area (in2)        RPM=revolutions per minute        T=torque (ft-lbf)        ROP=rate of penetration (ft/hr)        
The basis of MSE is that there is a measurable and calculable quantity of energy required to destroy a unit volume of subterranean rock. Operationally, this energy is delivered to the rock by rotating (torsional energy) and applying weight to (gravitational energy) a rock drill bit via the drill string. Historically, drilling efficiency could then be gauged by comparing the compressive strength of the rock against the quantity of energy used to destroy it.
Current drilling operations are regularly conducted in such a way that directly increases rate of penetration (ROP) of a rock drill bit through an environ. Traditional mechanical specific energy (MSE) theory posits that if one can minimize MSE while drilling, a resulting increase in ROP will be observed as is defined within the calculation of MSE. It is presently widely accepted by the oil and gas industry that even good drilling operations have a MSE efficiency factor of approximately 35%, i.e. only 35% of the energy put into the drilling operation actually goes towards destroying subterranean rock. While this initial 35% of MSE expenditure goes toward failing the subterranean rock, some portion of the remaining 65% of MSE is expended to collectively extrude crushed rock particles across the face of each cutting insert of a rock drill bit while drilling.
Prior efforts have been focused on developing resilient, high strength inserts having at least a polycrystalline diamond (“PCD”) cutting face that is designed for hard rock abrasion. There have been many advancements in fabrication processes associated with sintering the PCD layer onto a back-supporting substrate material, e.g.—tungsten carbide, of an insert, sorting of the diamond particles in the PCD layer, and general materials selection. However, improvements to the configuration of the cutting insert have largely been focused on increasing performance based on preserving traits derived from these advancements.
Thus, a need still exists for a cutting insert configuration that effectively reduces the mechanical specific energy that is expended to extrude crushed rock particles across the face of a cutting insert during drilling.